The prospect of controlling the wetting properties (so-called wetting control) of solid surfaces are currently attracting much focus. There are a vast number of envisaged applications, including bioscience (e.g. cell-growth control and selective adsorption of biomaterial), micro-fluid control (e.g. controllable gates and fluid propulsion) and printing techniques (e.g. updatable printing plates).
The wetting properties of a solid surface can be classified on a scale between lyophobic and lyophilic. A lyophilic surface is a surface that attracts liquids whereas a lyophobic surface rejects liquids. A sub-class of the lyophobic and lyophilic conception is denoted hydrophobic and hydrophilic, respectively, and is restricted to wetting properties for water only. Another sub-class of the lyophobic and lyophilic conception is denoted lipophobic and lipophilic, respectively, and is restricted to wetting properties for oils only (in fact, a lipophilic surface is typically hydrophobic at the same time, and vice versa).
The wetting properties can be measured as the contact angle θ of a liquid drop on a solid surface. The wetting properties exhibited by a liquid on a solid surface depend to a large degree on the surface tension γ experienced by the liquid on that surface. The contact angle θ of a droplet on a surface can be estimated using the Young-Dupré equationγsv=γslγlv cos θwhere γsv, γsl and γlv are the surface tensions of the solid-vapor, solid-liquid and liquid-vapor interface, respectively. Obviously, a large contact angle corresponds to a lyophobic surface and a small contact angle corresponds to a lyophilic surface.
There are two conceptually different approaches for controlling the wetting properties of a surface; either the intrinsic surface properties are changed (i.e. between lyophobic and lyophilic), or the liquid behavior is manipulated by electrostatic forces.
In the first case, changing a surface from lyophilic to lyophobic will generally result in liquid-repelling effect and changing a surface from lyophobic to lyophilic will generally result in a liquid-attracting effect. A number of alternative approaches have been suggested for controlling the intrinsic wetting properties, including temperature, light, and electrochemical and chemical reactions.
In the second case, the droplet is exposed to electrostatic forces that counteract the lyophilic or lyophobic forces of the surface and that thus move the droplet from a lyophilic surface towards a lyophobic surface, or vice versa.
A surface having reversibly switchable intrinsic wetting properties has been described in the article “A Reversably Switching Surface” (by Lahann et al, Science, vol 299, 17 Jan. 2003). Described therein is an approach for dynamically controlling interfacial wetting properties using conformational transitions of surface-confined molecules. This is attained using a self-assembled monolayer of (16 Mercapto)hexadecanoic acid on an aluminum substrate. The molecular layer acts as a surfactant, and each molecule is thus controllable between a hydrophobic and a moderately hydrophilic state. To this end the molecule arrangement was selected in order to ensure a sufficient spatial freedom for each molecule.
Each molecule in the monolayer contains a so-called anchor (the mercapto part) that is attached to a hydrophobic alkyl chain that is capped by a hydrophilic carboxylate group. In their pristine states, the molecules in the monolayer are pointing their hydrophilic carboxylate groups toward the outer surface, which thus exhibits a hydrophilic property. However, in case the aluminum substrate is positively charged, the carboxylate groups (negatively charged) are attracted toward the substrate and the hydrophobic alkyle chains are instead somewhat exposed, turning the surface moderately hydrophobic. In effect, the wetting properties of the surface can be controlled by means of an electric potential.
However, the technique described in the above article is somewhat restricted. First, there are severe restrictions on the molecule monolayer regarding the molecule properties as well as the molecule density. Second, the switching is dynamic in the sense that the wetting properties always return to the initial state upon removal of the electric potential. Third, the device described is not easily adapted for large-scale and cost-effective manufacturing since it requires fairly complicated manufacturing processes and since the material choices are severely restricted. Fifth, the surface switching is between hydrophobic and only moderately hydrophobic. Consequently, the technique is not applicable to applications requiring clearly hydrophilic surfaces.
It is therefore an object of the present invention to provide a wettability switch that facilitates cost-effective large-scale manufacturing. In addition, it is a general object of the present invention to provide a wettability switch that exhibits improved switching capabilities compared to prior known devices.